STEP 1: Navigate to the specific journey's page

STEP 2: You can send a mobile message from within the journey record page in two ways.
1) By clicking on the "Send Mobile Message" button.
2) By clicking on the "Action" dropdown and then selecting "Send Mobile Message."
STEP 3: Compose your mobile message in the pop-up window.

If you have templated messages created, you can select them from the "Template" dropdown.
STEP 4: Press Send !
By scrolling down to the activity area of your journey's record, you'll be able to view the send mobile message.